This invention relates to mass properties measurement devices wherein the measured parameter is the period of rotational oscillation of a torsional pendulum. In particular, it relates to an apparatus having means for measuring the moment of inertia of an object, such as the head of a golf club, about its axis of rotation, and for displaying the results of such a measurement.
It is well known that for a torsional pendulum in which the rotational axis lies parallel to the gravitational field, the moment of inertia of an object can be expressed as: ##EQU1## where .chi. is the torsional constant of the resilient member, and T is the period of oscillation.
The equation may be simplified to I=AT.sup.2 (II.), where A is a proportionality constant. Thus I is proportional to the square of the period of oscillation.
The relevant prior art describes measurement systems comprising a torsional pendulum, in which a rotationally oscillating fixture is loaded with the object under test, and a means of timing and displaying the period of oscillation. In addition, methods have been described for computing moment of inertia from such measurements.
U.S. Pat. Nos. 3,693,413 and 4,212,193 propose differing torsional systems, but utilize similar computational techniques, each requiring three distinct period measurements, and the simultaneous solution of three equations in order to determine the moment of inertia of a given test object.
The principal drawback to the previous methods is that none of them afford the user a means of making a direct measurement of moment of inertia. The difficulty with the prior methods lies in the fact that the torsional costant .chi., remains an unknown quantity in any single measurements, and must therefore be solved for by the means described in the prior art.
U.S. Pat. No. 4,212,193 proposes alternatively that moment of inertia could be read directly, utilizing the teachings set forth therein, if an arbitrary set of measurement units were adopted. However in recent years, industry has begun to publish moment of inertia specifications for products such as golf clubs in terms of conventional units. Consequently, measurements expressed in arbitrary units, such as those proposed in the prior art, cannot be compared easily to similar quantities expressed in conventional units.
Moreover, the apparatus described in U.S. Pat. No. 4,212,193 produces results that are linear with the period of oscillation, whereas moment of inertia by definition is proportional to the square of the period. The resulting error in the measured value is then proportional to the percent difference between said measured value and that of the calibration standard. More importantly, any calculation based on said measurement would propagate the error.
It is the principal object of the present invention to provide a convenient means for measuring the moment of inertia of an object by (1) measuring the period of the rotationally oscillating system on which said object is loaded, (2) converting said period into a number equal to the moment of inertia of a rotationally oscillating mass, expressed in conventional units, and (3) displaying said number so that the quantity may be evaluated.